Re: [PATCH RFC] extent mapped page cache

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2007-07-24 at 16:13 -0400, Trond Myklebust wrote:
> On Tue, 2007-07-24 at 16:00 -0400, Chris Mason wrote:
> > On Tue, 10 Jul 2007 17:03:26 -0400
> > Chris Mason <chris.mason@oracle.com> wrote:
> > 
> > > This patch aims to demonstrate one way to replace buffer heads with a
> > > few extent trees.  Buffer heads provide a few different features:
> > > 
> > > 1) Mapping of logical file offset to blocks on disk
> > > 2) Recording state (dirty, locked etc)
> > > 3) Providing a mechanism to access sub-page sized blocks.
> > > 
> > > This patch covers #1 and #2, I'll start on #3 a little later next
> > > week.
> > > 
> > Well, almost.  I decided to try out an rbtree instead of the radix,
> > which turned out to be much faster.  Even though individual operations
> > are slower, the rbtree was able to do many fewer ops to accomplish the
> > same thing, especially for merging extents together.  It also uses much
> > less ram.
> 
> The problem with an rbtree is that you can't use it together with RCU to
> do lockless lookups. You can probably modify it to allocate nodes
> dynamically (like the radix tree does) and thus make it RCU-compatible,
> but then you risk losing the two main benefits that you list above.

I thought on this, and I came to the conclusion that the tree rotations
used to balance binary trees are incompatible with RCU. The rotation can
hide one branch. Hence I started writing a B+tree that is RCU compatible
much like the Radix tree.

Current code here:
  http://programming.kicks-ass.net/kernel-patches/vma_lookup/btree.patch

Still needs some work, but is usable.